Polyvinyl chloride resins have high flame retardancy, excellent resistance to chemicals, high rigidity and many other desirable properties, but suffer from low impact strength and heat distortion resistance.
In addition, it is impossible to obtain satisfactory formings from sheet materials of polyvinyl chloride resins by the vacuum or pressure forming technique because of lack in good balance of tensile strength and stretching characteristics at high temperatures.
When a sheet material of a polyvinyl chloride resin is subjected to vacuum or pressure forming at 100.degree. to 130.degree. C., the material undergoes some elongation but its tensile strength is still too high, thus causing various problems such as poor moldability at corner portions, large shrinkage after forming, and impracticability of deep drawing. When the forming temperature is raised to 140.degree. C. or higher, the tensile strength decreases but stretchability is lost almost completely; the result is that satisfactory vacuum or pressure forming cannot be effected.
Such a difficulty in effecting vacuum and pressure forming, combined with poor heat distortion resistance, has made it difficult to use a polyvinyl chloride resin as a material for housing of electric and electronic parts and for many other applications.
Post-chlorination is known as a means for improving the heat distortion resistance of polyvinyl chloride resins. The resulting resin, however, has high melt viscosity and is difficult to mold. In addition, stretching characteristics of polyvinyl chloride resin at elevated temperatures cannot be improved by this modification. Thus chlorinated polyvinyl chloride resins are not suitable for vacuum and pressure forming.
It is also known that the heat distortion resistance of polyvinyl chloride resins can be enhanced by admixing appropriate amounts of chlorinated polyvinyl chloride resins. But no improvement in vacuum and pressure forming properties can be achieved by this method, too.
Other techniques to improve the thermal resistance of polyvinyl chloride resins were proposed in Japanese Patent Publication No. 6361/1964, in which a polyvinyl chloride resin is blended with poly-.alpha.-methylstyrene or a copolymer consisting mainly of .alpha.-methylstyrene, but no description can be found in the above-cited patent about impact strength and thermoformability of the resulting blend. It is generally accepted, as described in Japanese Patent Publication Nos. 24867/1970, 29895/1976 and 31818/1976 that this type of blend has the following disadvantages: As the content of .alpha.-methylstyrene in copolymers increases, compatibility with the polyvinyl chloride resin tends to lower, resulting in poor rigidity and other mechanical properties; extrusion or calendering of the copolymers fail to give sheets of good quality; and no improvement in thermal resistance and impact strength can be expected.
A large variety of techniques have been proposed for improvement of impact strength of polyvinyl chloride resins, but any of these, although effective in enhancing impact strength, tends to adversely affect heat distortion resistance and is of no use for the improvement of vacuum or pressure forming properties.
In an attempt to overcome these difficulties associated with polyvinyl chloride resins, the present inventors have found that incorporating a proper amount of an .alpha.-methylstyrene/acrylonitrile copolymer improves the heat distortion resistance of polyvinyl chloride resins, and serves to lower the tensile strength and enhance stretching characteristics at high temperatures, thus giving excellent sheet materials with significantly improved vacuum and pressure forming properties without deterioration of extrusion, calendering and other molding properties of the polyvinyl chloride resins.